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<p>C <a href="type.html" title="c/language/type" class="mw-redirect">类型系统</a>中每一个独立的类型在都有数个该类型的<i>限定</i>版本，对应 <a href="const.html" title="c/language/const">const</a> 、 <i>volatile</i> 及限定对于指向对象指针的 <a href="restrict.html" title="c/language/restrict">restrict</a> 限定符中的一个、两个或全部三个。此页面描述 <i>volatile</i> 限定符的效果。
</p><p>每一个通过对 volatile 限定类型左值表达式的访问（读与写），对于优化意图都被认为是可观副效应，从而访问会严格按照抽象机的规则求值（即所有写入会在下一个序点之前的某时完成）。这表明在执行的单个线程内，volatile 访问不能被优化掉，亦不能与另一个被<a href="eval_order.html" title="c/language/eval order">顺序点</a>分隔了 volatile 访问的可观副效应重排。
</p><p>一个非 volatile 值到 volatile 值的转换是无效果的。欲使用 volatile 语义访问非 volatile 对象，必须先将其地址转换成指向 volatile 类型的指针，再通过该指针访问该对象。
</p><p>任何通过非 volatile 左值结果，对拥有 volatile 限定类型的对象尝试读或写会导致未定义行为：
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="c source-c"><pre class="de1"><span class="kw4">volatile</span> <span class="kw4">int</span> n <span class="sy1">=</span> <span class="nu0">1</span><span class="sy4">;</span> <span class="co1">// volatile 限定类型</span>
<span class="kw4">int</span><span class="sy2">*</span> p <span class="sy1">=</span> <span class="br0">(</span><span class="kw4">int</span><span class="sy2">*</span><span class="br0">)</span><span class="sy3">&amp;</span>n<span class="sy4">;</span>
<span class="kw4">int</span> val <span class="sy1">=</span> <span class="sy2">*</span>p<span class="sy4">;</span> <span class="co1">// 未定义行为</span></pre></div></div>
<p>volatile 限定的结构体或联合体类型，其成员会获取其所属类型的限定（当通过 <code><b>.</b></code> 或 <code><b>-&gt;</b></code> 运算符时）：
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="c source-c"><pre class="de1"><span class="kw1">struct</span> s <span class="br0">{</span> <span class="kw4">int</span> i<span class="sy4">;</span> <span class="kw4">const</span> <span class="kw4">int</span> ci<span class="sy4">;</span> <span class="br0">}</span> s<span class="sy4">;</span>
<span class="co1">// s.i 类型是 int，s.ci 的类型是 const int</span>
<span class="kw4">volatile</span> <span class="kw1">struct</span> s vs<span class="sy4">;</span>
<span class="co1">// vs.i 和 vs.ci 的类型各是 volatile int 和 const volatile int</span></pre></div></div>
<p>若数组类型声明具有 volatile 类型限定符（通过使用<a href="typedef.html" title="c/language/typedef">typedef</a>），则数组类型本身不是 volatile 限定的，但其元素类型是 volatile 限定。若函数类型声明具有 volatile 类型限定（通过使用 <a href="typedef.html" title="c/language/typedef">typedef</a> ），则行为未定义。
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="c source-c"><pre class="de1"><span class="kw1">typedef</span> <span class="kw4">int</span> A<span class="br0">[</span><span class="nu0">2</span><span class="br0">]</span><span class="br0">[</span><span class="nu0">3</span><span class="br0">]</span><span class="sy4">;</span>
<span class="kw4">volatile</span> A a <span class="sy1">=</span> <span class="br0">{</span><span class="br0">{</span><span class="nu0">4</span>, <span class="nu0">5</span>, <span class="nu0">6</span><span class="br0">}</span>, <span class="br0">{</span><span class="nu0">7</span>, <span class="nu0">8</span>, <span class="nu0">9</span><span class="br0">}</span><span class="br0">}</span><span class="sy4">;</span> <span class="co1">// volatile int 的数组的数组</span>
<span class="kw4">int</span><span class="sy2">*</span> pi <span class="sy1">=</span> a<span class="br0">[</span><span class="nu0">0</span><span class="br0">]</span><span class="sy4">;</span> <span class="co1">// 错误：a[0] 拥有 volatile int* 类型</span></pre></div></div>
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<p>在函数声明中，关键词<code>volatile</code>可以出现于方括号内，用以声明函数参数的数组类型。它对数组所转换得的指针类型赋予限定。
</p><p>下列两个声明声明同一函数：
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="c source-c"><pre class="de1"><span class="kw4">void</span> f<span class="br0">(</span><span class="kw4">double</span> x<span class="br0">[</span><span class="kw4">volatile</span><span class="br0">]</span>, <span class="kw4">const</span> <span class="kw4">double</span> y<span class="br0">[</span><span class="kw4">volatile</span><span class="br0">]</span><span class="br0">)</span><span class="sy4">;</span>
<span class="kw4">void</span> f<span class="br0">(</span><span class="kw4">double</span> <span class="sy2">*</span> <span class="kw4">volatile</span> x, <span class="kw4">const</span> <span class="kw4">double</span> <span class="sy2">*</span> <span class="kw4">volatile</span> y<span class="br0">)</span><span class="sy4">;</span></pre></div></div>
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<p>指向非 volatile 类型的指针可以隐式转换成指向同一或<a href="type.html#.E5.85.BC.E5.AE.B9.E7.B1.BB.E5.9E.8B" title="c/language/types">兼容类型</a>的 volatile 限定版本的指针。逆向转换可以由类型转换表达式进行。
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="c source-c"><pre class="de1"><span class="kw4">int</span><span class="sy2">*</span> p <span class="sy1">=</span> <span class="nu0">0</span><span class="sy4">;</span>
<span class="kw4">volatile</span> <span class="kw4">int</span><span class="sy2">*</span> vp <span class="sy1">=</span> p<span class="sy4">;</span> <span class="co1">// OK：添加限定符（ int 到 volatile int）</span>
p <span class="sy1">=</span> vp<span class="sy4">;</span> <span class="co1">// 错误：丢弃限定符（ volatile int 到 int）</span>
p <span class="sy1">=</span> <span class="br0">(</span><span class="kw4">int</span><span class="sy2">*</span><span class="br0">)</span>vp<span class="sy4">;</span> <span class="co1">// OK：类型转换</span></pre></div></div>
<p>注意指向<code>T</code>的二重不可转换成指向<code>volatile T</code>的二重指针；对于要兼容的两个类型，它们的限定必须相同：
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="c source-c"><pre class="de1"><span class="kw4">char</span> <span class="sy2">*</span>p <span class="sy1">=</span> <span class="nu0">0</span><span class="sy4">;</span>
<span class="kw4">volatile</span> <span class="kw4">char</span> <span class="sy2">**</span>vpp <span class="sy1">=</span> <span class="sy3">&amp;</span>p<span class="sy4">;</span> <span class="co1">// 错误： char* 和 volatile char* 不是兼容类型</span>
<span class="kw4">char</span> <span class="sy2">*</span> <span class="kw4">volatile</span> <span class="sy2">*</span>pvp <span class="sy1">=</span> <span class="sy3">&amp;</span>p<span class="sy4">;</span> <span class="co1">// OK，添加限定符（ char* 到 char* volatile ）</span></pre></div></div>
<h3><span class="mw-headline" id="volatile_.E7.9A.84.E7.94.A8.E6.B3.95"><code>volatile</code> 的用法</span></h3>
<div class="t-li1"><span class="t-li">1)</span> <a href="storage_duration.html" title="c/language/storage duration">static</a> <code>volatile</code> 对象模仿映射于内存的 I/O 端口，而 <code>static</code> <code>const</code> <code>volatile</code> 对象模仿映射于内存的输入端口，例如实时时钟：
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="c source-c"><pre class="de1"><span class="kw4">volatile</span> <span class="kw4">short</span> <span class="sy2">*</span>ttyport <span class="sy1">=</span> <span class="br0">(</span><span class="kw4">volatile</span> <span class="kw4">short</span><span class="sy2">*</span><span class="br0">)</span>TTYPORT_ADDR<span class="sy4">;</span>
<span class="kw1">for</span><span class="br0">(</span><span class="kw4">int</span> i <span class="sy1">=</span> <span class="nu0">0</span><span class="sy4">;</span> i <span class="sy1">&lt;</span> N<span class="sy4">;</span> <span class="sy2">++</span>i<span class="br0">)</span>
    <span class="sy2">*</span>ttyport <span class="sy1">=</span> a<span class="br0">[</span>i<span class="br0">]</span><span class="sy4">;</span> <span class="co1">// *ttyport 是 volatile short 类型的左值</span></pre></div></div></div>
<div class="t-li1"><span class="t-li">2)</span> <span class="t-lc"><a href="../program/sig_atomic_t.html" title="c/program/sig atomic t">sig_atomic_t</a></span> 类型的 <code>static</code> <code>volatile</code> 对象用于与 <span class="t-lc"><a href="../program/signal.html" title="c/program/signal">signal</a></span> 处理交流。</div>
<div class="t-li1"><span class="t-li">3)</span> 含有对 <span class="t-lc"><a href="../program/setjmp.html" title="c/program/setjmp">setjmp</a></span> 宏调用的函数中的局部 <code>volatile</code> 变量，是在 <span class="t-lc"><a href="../program/longjmp.html" title="c/program/longjmp">longjmp</a></span> 返回后仅有保证恢复其值的局部变量。</div>
<div class="t-li1"><span class="t-li">4)</span> 另外，volatile 变量可用于禁用某些优化形式，例如禁用死存储消除，或为微基准禁用常量折叠。</div>
<p>注意 volatile 变量不适合线程间交流；它们不提供原子性、同步或内存顺序。读取一个被另一线程未经同步地修改的 volatile 变量，或两个未同步的线程的共时修改，对于一些数据竞争是未定义行为。
</p>
<h3><span class="mw-headline" id=".E5.85.B3.E9.94.AE.E8.AF.8D">关键词</span></h3>
<p><a href="../keyword/volatile.html" title="c/keyword/volatile"><tt>volatile</tt></a>
</p>
<h3><span class="mw-headline" id=".E7.A4.BA.E4.BE.8B">示例</span></h3>
<div class="t-example"><p>展示用 volatile 禁用优化</p><div class="t-example-live-link"><div class="coliru-btn coliru-btn-run-init">运行此代码</div></div>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="c source-c"><pre class="de1"><span class="co2">#include &lt;stdio.h&gt;</span>
<span class="co2">#include &lt;time.h&gt;</span>
 
<span class="kw4">int</span> main<span class="br0">(</span><span class="kw4">void</span><span class="br0">)</span>
<span class="br0">{</span>
    <a href="../chrono/clock_t.html"><span class="kw525">clock_t</span></a> t <span class="sy1">=</span> <a href="../chrono/clock.html"><span class="kw514">clock</span></a><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
    <span class="kw4">double</span> d <span class="sy1">=</span> <span class="nu16">0.0</span><span class="sy4">;</span>
    <span class="kw1">for</span> <span class="br0">(</span><span class="kw4">int</span> n<span class="sy1">=</span><span class="nu0">0</span><span class="sy4">;</span> n<span class="sy1">&lt;</span><span class="nu0">10000</span><span class="sy4">;</span> <span class="sy2">++</span>n<span class="br0">)</span>
       <span class="kw1">for</span> <span class="br0">(</span><span class="kw4">int</span> m<span class="sy1">=</span><span class="nu0">0</span><span class="sy4">;</span> m<span class="sy1">&lt;</span><span class="nu0">10000</span><span class="sy4">;</span> <span class="sy2">++</span>m<span class="br0">)</span>
           d <span class="sy2">+</span><span class="sy1">=</span> d<span class="sy2">*</span>n<span class="sy2">*</span>m<span class="sy4">;</span> <span class="co1">// 读写非 volatile 对象 </span>
    <a href="../io/fprintf.html"><span class="kw851">printf</span></a><span class="br0">(</span><span class="st0">"Modified a non-volatile variable 100m times. "</span>
           <span class="st0">"Time used: %.2f seconds<span class="es1">\n</span>"</span>,
           <span class="br0">(</span><span class="kw4">double</span><span class="br0">)</span><span class="br0">(</span><a href="../chrono/clock.html"><span class="kw514">clock</span></a><span class="br0">(</span><span class="br0">)</span> <span class="sy2">-</span> t<span class="br0">)</span><span class="sy2">/</span><a href="http://zh.cppreference.com/w/c/chrono/CLOCKS_PER_SEC"><span class="kw522">CLOCKS_PER_SEC</span></a><span class="br0">)</span><span class="sy4">;</span>
 
    t <span class="sy1">=</span> <a href="../chrono/clock.html"><span class="kw514">clock</span></a><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
    <span class="kw4">volatile</span> <span class="kw4">double</span> vd <span class="sy1">=</span> <span class="nu16">0.0</span><span class="sy4">;</span>
    <span class="kw1">for</span> <span class="br0">(</span><span class="kw4">int</span> n<span class="sy1">=</span><span class="nu0">0</span><span class="sy4">;</span> n<span class="sy1">&lt;</span><span class="nu0">10000</span><span class="sy4">;</span> <span class="sy2">++</span>n<span class="br0">)</span>
       <span class="kw1">for</span> <span class="br0">(</span><span class="kw4">int</span> m<span class="sy1">=</span><span class="nu0">0</span><span class="sy4">;</span> m<span class="sy1">&lt;</span><span class="nu0">10000</span><span class="sy4">;</span> <span class="sy2">++</span>m<span class="br0">)</span>
           vd <span class="sy2">+</span><span class="sy1">=</span> vd<span class="sy2">*</span>n<span class="sy2">*</span>m<span class="sy4">;</span> <span class="co1">// 读写 volatile 对象</span>
    <a href="../io/fprintf.html"><span class="kw851">printf</span></a><span class="br0">(</span><span class="st0">"Modified a volatile variable 100m times. "</span>
           <span class="st0">"Time used: %.2f seconds<span class="es1">\n</span>"</span>,
           <span class="br0">(</span><span class="kw4">double</span><span class="br0">)</span><span class="br0">(</span><a href="../chrono/clock.html"><span class="kw514">clock</span></a><span class="br0">(</span><span class="br0">)</span> <span class="sy2">-</span> t<span class="br0">)</span><span class="sy2">/</span><a href="http://zh.cppreference.com/w/c/chrono/CLOCKS_PER_SEC"><span class="kw522">CLOCKS_PER_SEC</span></a><span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span></pre></div></div>
<p>可能的输出：
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="text source-text"><pre class="de1">Modified a non-volatile variable 100m times. Time used: 0.00 seconds
Modified a volatile variable 100m times. Time used: 0.79 seconds</pre></div></div> 
</div>
<h3><span class="mw-headline" id=".E5.BC.95.E7.94.A8">引用</span></h3>
<div class="t-ref-std-11">
<ul><li> C11 standard (ISO/IEC 9899:2011): 
</li></ul>
<dl><dd><ul><li> 6.7.3 Type qualifiers (p: 121-123)
</li></ul>
</dd></dl>
<div class="t-ref-std-c99">
<ul><li> C99 standard (ISO/IEC 9899:1999): 
</li></ul>
<dl><dd><ul><li> 6.7.3 Type qualifiers (p: 108-110)
</li></ul>
</dd></dl>
<div class="t-ref-std-c89">
<ul><li> C89/C90 standard (ISO/IEC 9899:1990): 
</li></ul>
<dl><dd><ul><li> 3.5.3 Type qualifiers 
</li></ul>
</dd></dl>
</div>
<h3><span class="mw-headline" id=".E5.8F.82.E9.98.85">参阅</span></h3>
<table class="t-dsc-begin">

</table></div>
</div>

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